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6-1
Oceanography
Composition of the Oceans:
Study of the Earth’s oceans.
By weight, seawater contains ~3.5% materials other
than water.
Draws upon a variety of disciplines.
Most of these are salts, predominantly sodiumchloride (table salt).
70% of the Earth’s surface is covered by oceans.
Many of these minerals brought to the oceans from
land sediments.
Clearly the oceans play an important role in the Earth
system as a whole.
Volcanoes also release gases into the oceans.
While the average salinity of the oceans is 35 parts
per thousand (35 ‰) it varies significantly.
Salinity is relatively high in regions with significant
evaporation.
Where would we expect this to occur?
What would we expect in regions of heavy rainfall?
What about regions of large inflow of water from the
continents?
Layers in the Oceans
The ocean can be vertically split into three major
units:
Near the surface is a region heated by sunlight.
Turbulence in the water tends to mix this layer well
leading to a nearly constant temperature of around
21–26°C.
Below this layer is a transition zone where
temperatures fall rapidly.
The final layer comprising about 80% of the ocean is
a deep layer at ~4°C.
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Topography of the Ocean Floor
While some research had been done prior to WWII
most of the sea floor remained a mystery prior to
that time.
They found it is not flat as they had supposed.
They found a long mountain range running down the
middle of the Atlantic ocean.
The mid-Atlantic ridge.
Geologists assumed it to be an essentially flat
featureless place.
However, WWII led to a new urgency to develop
technology to detect underwater objects.
It is now known that this ridge is much more
extensive and winds its way over 40,000 miles
through all the Earth’s oceans.
Now called the mid-Oceanic ridge.
New technology, some developed during the war,
allowed geologists to examine the sea floor.
As we shall see later, this discovery helped lead to
the revival of the idea of continental drift and plate
tectonics.
Active Plate Margins
The mid-oceanic ridge is thought to be a region of
current formation of new crust.
What I have just described is what is seen at passive
plate margins where there is no motion between the
continent and oceanic plates.
Plates are forced apart in these regions.
For example, the coasts along the Atlantic ocean.
Very distinctive double ridged structure to the midoceanic ridge:
However, the topography is quite different where the
plates are converging. For example around much of
the Pacific.
These are active plate margins.
Not like features seen in mountain ranges on land.
6-3
At such boundaries the continental shelf is greatly
reduced to practically nonexistent.
Instead such margins are marked by deep-ocean
trenches.
These are the deepest portions of the ocean and can
extend to depths of over 11 km.
Associated with substantial geologic activity
(earthquakes, volcanoes, and mountain building).
Other Features of the Sea Floor
In addition to the features just mentioned some
additional features are often seen.
Underwater volcanism is seen at a number of
locations.
Some of these volcanoes become big enough to poke
above sea level to become volcanic islands.
Examples?
However, sometimes they don’t break the surface.
The ocean floor is dotted by a number of these
volcanoes which are called sea mounts.
Oftentimes chains of these volcanoes are seen.
Coral Reefs and Atolls
Coral reefs are constructed primarily from the
skeletal remains of corals and algae.
Often these features will form rings surrounding a
central lagoon called atolls.
Associated with volcanism at hot spots.
Creatures which create coral reefs only exist near the
surface and in warm waters.
Once formed, a volcanic island will eventually be
eroded to near sea level.
These may then become submerged as they move to
deeper locations.
These form distinctive flat-topped features called
guyots.
Thus most coral reefs are found in warm waters,
particularly in the Indian and Pacific oceans.
Atolls thought to be formed around volcanic islands.
As the island submerges, coral continues to build
upon the preexisting coral.
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Composition of the Sea Floor
Origin of Sea Floor Sediments
Oceanic crust is continually being created at the mid- The sea floor sediments are derived from a variety of
sources but most ultimately derive from land.
oceanic ridge.
Mid-oceanic volcanism results mostly in the rock
basalt and its intrusive cousin gabbro.
Terrigenous Sediments
These sediments are derived directly from land.
Thus the oceanic crust consists almost entirely of
these rocks.
They consist mostly of small mud-sized solid
particles created from the weathering of rocks.
The crust is on average about 7km thick with a 2km
thick layer of basalt atop ~5km of gabbro.
Carried to the ocean by rivers or other media.
These rocks are overlain by a thin (~200m) layer of
sediments.
They eventually settle out onto the sea floor.
Biogenous Sediments
Hydrogenous Sediments
Some sediments are created by marine life.
Some sediments precipitate directly from the water
Creatures take material out of solution in the water
and use it for their own purposes creating shells and
skeletons and other hard parts.
Examples include some calcium carbonates and
silica (SiO2).
When these creatures die they rain down on the sea
floor to become deposits.
Most of this material and the material used by sea
creatures was derived from land.
Thus, ultimately, these sediments have their source
on land as well.
6-5
Ocean Currents
Surface currents on the oceans are driven
predominantly by winds.
Deep Ocean Currents
In addition to the surface currents, the deep water
undergoes circulation as well.
Friction between the air and water transfers energy
from the air to the water.
When air blows over the water steadily along a given
direction it sets ocean currents in motion.
Winds are also responsible for creating most of the
ocean waves we see.
If winds really do drive ocean currents, what pattern
in the ocean currents would we expect to see?
These currents are driven by density differences in
the water.
Increased salinity raises the density of water.
Decreased temperature has the same effect.
Salty, cold water will be relatively dense and may
sink towards the ocean bottom.
As water sinks it pulls more water after it.
Water at depth then flows back to the origin of the
subsiding water setting up a circulation pattern.
When ocean currents change, substantial changes in
the climate can result.
For example, at times water from near the equator
flows towards high northern latitudes.
Ocean Currents and Climate
As it flows through the region of the subtropical
high it suffers evaporation increasing salinity.
Ocean currents (both surface and deep water) can
have a great influence on climate.
However it is still warm so it doesn’t sink.
The air circulation pattern transports the most energy
from the equator towards the poles.
When it reaches the colder more northerly regions it
cools down and eventually sinks.
However, ocean currents play an important role as
well (transporting ~25% of the total).
Water then returns at depth.
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Shore and Coastal Processes
Coastal regions are amongst the most popular.
Water Waves
Most of the energy which is available to work on
shores is brought by water waves.
People flock to beaches for relaxation.
Beachfront property is highly sought after.
However, coastal regions are quite fragile.
Most water waves are generated by winds blowing
over the ocean surface.
Size of waves will be greatest where winds blow
from a constant direction over long distances.
Our actions can have unintended consequences.
Understanding the processes involved can help us
avoid costly mistakes.
Also of geologic interest.
Characteristics of Waves
A water wave can be characterized by its height,
wavelength, and period.
The height of a wave is the vertical distance between
adjacent wave crests and troughs.
Given this, in which ocean would you expect to see
the largest waves?
In which hemisphere?
The period of a wave is the time it takes a wave to
pass a stationary point.
What are typical periods for a wave?
The velocity of a wave is related to the wavelength
and period by:
A wave’s energy is determined by its height.
Typical wave heights in the open ocean are a few
meters.
The wavelength of a wave is the distance (crest to
crest or trough to trough) between two waves.
Typical wavelengths are 40 to 400 meters.
velocity =
wavelength
Period
What are typical velocities of water waves?
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Wave Motion at the Shore
Waves may travel for great distances.
Eventually the water at the top of the wave overruns
the water lower down.
The wave “breaks.”
However, when they approach a shore the ocean
depth becomes shallower and shallower.
The water now moves towards the shore as surf.
Eventually the wave motion begins to be restricted by
the ocean bottom:
It is in the surf region where the energy of the wave
is expended.
friction starts to slow the wave down.
Energy is used to erode and transport material.
The wavelength decreases and the height starts to
increase.
Coastal Erosion
If the sea floor slopes gently waves will break farther
offshore.
Waves battering the shore can break up and loosen
coastal rocks.
Much of its energy will then be dissipated away
from the shore.
The amount of erosion which takes place depends on
what?
Where the sea floor is steeply sloped (e.g. at active
plate margins) waves may hit the shore with full
force.
Wave action will tend to straighten out coast lines.
To see why consider what happens as a wave
approaches a jagged shore...
6-8
Coastal Transport and Deposition
Where does the sand come from?
Much like rivers or wind, wave action can result in
transport of material.
Material is often transported by the longshore
current.
Often times the material being transported is sand
sized particles forming a beach.
Beaches form a relatively narrow zone where wave
action takes place.
Formed of material being transported.
If not disturbed by outside forces, a beach will
eventually reach an equilibrium.
Deposition and Depositional Landforms
Material from rivers or other sources is brought to
the system.
As with rivers or wind, coastal processes will deposit
material when it is no longer capable of transporting
it.
Material is then transported away by coastal action
This can occur for a variety of reasons:
When these are equal the beach has reached an
equilibrium and will remain stable.
1. Wind velocity and wave action may vary
seasonally.
Disrupt the system though and the beach may
quickly erode away—or grow.
Often, wave action is more vigorous in winter with
increased storm activity, less so in summer:
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2. Water depth increases abruptly—waves don’t
break in region—longshore current is disrupted.
This can occur at the entrance to a bay. What
happens?
Human-induced Erosion and
Deposition
Beaches left to their own devices form a delicate
balance betwen material deposited and removed.
Our actions can disrupt this process whether
intentionally or not.
It does not even have to be at the coast:
For example: damming a river upstream may cut off
a rivers supply of sediment.
Without new sediment brought in to beaches what
will happen to the beaches?
Breakwaters:
Groins
Sometimes structures are built parallel to the shore to
intercept and dissipate wave energy. How?
Groins are features built out perpendicular from the
shore.
How do groins work?
With less wave energy hitting the shore what would
you expect to happen?
What happens downcurrent?
Might be good if one is trying to protect a beach.
Not so good if one is trying to protect a harbor
which now gets clogged up with sand.
Not very neighborly!